This invention relates to a measurement of the range of a source of electromagnetic radiation and, more particularly, to the use of passive ranging by examination of relative attenuation among a plurality of spectral lines wherein differences in attenuation among various portions of the radiation spectrum arise from selective atmospheric absorption of radiation at various frequencies as a function of propagation distance of the radiation through the atmosphere. The foregoing attenuation is in addition to the attenuation arising from the spreading of the waves of radiation through increasing regions of space, the latter attenuation following the well-known relationship of intensity varying as the inverse square of the range from a point source of the radiation.
Various objects, such as the plume of a rocket or other fire, or a hot filament or gas discharge of a lamp, are known to act as sources of radiation having characteristic spectra. There are situations in which it is desirable to determine the location of such a source from a viewing site distant from the source, the location data including range, elevation and azimuth of the target source from the viewing site. However, a problem arises in that the usual apparatuses for determination of target location, such as active radar, are not operative with the foregoing type of radiant energy signal for a passive determination of the range of the source. A further problem arises when a target conceals its source of radiation or when the source of radiation is inactive.
The aforementioned problem is overcome and other advantages are provided by a system and method of passive ranging, in accordance with the invention, wherein a suitable target, or distant source of radiation, is identified by its electromagnetic spectrum during a target acquisition procedure and, thereafter, the spectrum of the radiation is analyzed to determine the effects of atmospheric attenuation on various parts of the spectrum. In the practice of the invention, prior knowledge of the spectrum, as emitted by the target, is employed in both the acquisition and the analysis stages. The invention is particularly useful in the situation wherein a source of radiation, on or near the ground, illuminates a cloud above the source, and a distant observer obtains range of the source by observation of radiation scattered from the cloud.
A typical spectrum includes both a continue distribution of spectral energies in an emission band or in each of a plurality of emission bands, as well as a line spectrum wherein individual ones of the lines are characteristic of certain constituent substances in a source of the radiation, such as the various gasses in a rocket plume. In accordance with the theory of the invention, a source of radiation, such as a rocket plume, emits radiation characterized by a known set of spectral emission lines and/or emission bands. The lines of the line spectrum, as well as an amplitude profile of the continuous spectrum, are useful in identifying the source of the radiation. Generally, the spectrum of a received radiation signal will be shifted in frequency by a Doppler shift due to motion of the source, and there will be a broadening of one or more of the spectral lines due to movement of the gasses and particles thereof within the rocket plume. To identify the spectrum of a received radiation signal automatically, as by use of a computer or other signal processor, the received spectrum may be correlated against known spectra from a set of previously stored spectra. The previously stored spectra correspond to respective ones of known rocket plumes and other sources of radiation which may be of interest. A match is obtained between the received spectrum and one of the known spectra, the match serving to identify the source of the radiation. The correlation also indicates a frequency offset between the two matching spectra and, hence, is useful in providing the additional information of Doppler shift.
In accordance with a feature of the invention, a continuous portion of the received spectrum can be employed to determine range of a target, such as the plume of a rocket, independently of whether or not there be any Doppler frequency shift. Operation of the invention to obtain the range may be explained as follows. As the radiation propagates through the atmosphere from the source to optical receiving apparatus employed by the invention, there may be interaction between the radiation and various substances dependent on the frequency of the radiation. The interaction results in a relative attenuation of various spectral components by the atmosphere as a function of frequency and a function of distance of propagation of the radiation through the atmosphere. Thus, the attenuation is indicative of target range.
Measurement of the ratios of intensities of radiations at the various spectral bands at a distance from the source will differ from the same measurements performed at the location of the source because of the selective absorption of the radiation at its various spectral bands. In the practice of the invention, a correlation is made between variation of an intensity ratio of any two spectral lines as a function of distance between source and the receiving apparatus. The range to the source is thereby obtainable from spectrometric measurements of the radiation, computation of the intensity ratio, and association of the specific range with a specific intensity ratio, or an average value of ranges obtained from sets of intensity ratios. A succession of range measurements may be differentiated to obtain range rate.
Another aspect of the present invention uses reflected solar radiation, or other radiation source (e.g., tunable laser, search light) located at a known position in the atmosphere, to determine the range and/or rate of a target. In the case of using the sun as the source, a reference measurement of solar radiation is made at a spot adjacent (i.e., proximal target) to an optical apparatus employing the present invention. The reflected solar radiation is also measured as a reflection off the target (i.e., distal target). As the reflected solar radiation propagates through the atmosphere from the target to optical receiving apparatus employed by the invention, there may be interaction between the solar radiation and various substances dependent on the frequency of the reflected solar radiation. The interaction results in a relative attenuation of various spectral components by the atmosphere as a function of optical frequency and a function of distance of propagation and look angle or orientation of the reflected solar radiation through the atmosphere. Thus, the attenuation is indicative of target range. In an alternative embodiment, because the position of the radiation source, adjacent spot, and atmosphere between the radiation source and radiation source adjacent spot are known, the reflected radiation from the adjacent spot can be calculated rather than measured.